Method of etching magnesium and magnesium base alloys



United States Patcnt Office 3,067,080 Patented Dec. 4, 1962 This invention relates to etching magnesium and magnesium base alloys. More particularly it relates to novel etching compositions and an improved method for etching these metals whereby there is produced a bright uniformly etched surface free of defects such as grooves and pitting.

In the fabrication of articles made of magnesium or magnesium base alloys an etching step is frequently required. This may be a mild or light surface etch used to prepare the article for subsequent treatments such as plating, painting, and the like, or for giving the article a certain desired appearance. A more severe type of etching, known as chemical milling, is used to produce articles of magnesium having a desired configuration by chemically removing the undesired portions of a blank of the metal.

Magnesium is usually etched in a bath comprising an aqueous solution of a mineral acid. Among the acids which can be used, sulfuric acid is a logical choice because of its low cost, low volatility, and relative ease of handling. Unfortunately, however, sulfuric acid solutions react quite violently with magnesium and magnesium base alloys, the etched surface produced thereby being nonuniform and thus unacceptable. Moreover, in addition to the nonuniform surface, prolonged etching in sulfuric acid solutions, as in chemical milling, produces severe tapering, i.e., a form of nonuniform etching in which the amount of metal removed from the article by the bath varies with the depth of immersion. Thus, for example, if a fiat magnesium panel of uniform thickness is suspended vertically in a sulfuric acid etching bath for a time to permit substantial etching to occur, it will be found that the portion of the panel closest to the surface of the solution is thicker than that which is farthest from the tjurface. Although this variation in rate of etching with depth of immersion is not significant when only a light surface etch is desired, it is highly objectionable in chemical milling operations.

It is possible to reduce the violence of the reaction between sulfuric acid and magnesium by continuously cooling the etching solution. This expedient, however, requires the use of expensive refrigeration equipment and in addition significantly reduces the over-all rate of etching, both of which factors markedly increase the operating costs of the process.

It has been discovered that magnesium and magnesium base alloys can be etched successfully by an improved method wherein the surface of the metal is treated with a hot aqueous solution comprising sulfuric acid and a small amount of a sulfonated material described in greater detail hereinafter. The method of the invention results in etched surfaces which are uniformly smooth and bright, and substantially free of tapering, groove formation, and pitting. At the same time a high rate of etching i possible because of the elevated temperatures which can be used in the process.

The sulfonated material used in accordance with this invention is selected from the class consisting of xylene sulfonic acid, toluene sulfonic acid, alkyl aryl polyether sulfonic acids, sulfonated oleic acid, the soluble metal salts of said acids, and mixtures thereof. These materials are incorporated in the aqueous sulfuric acid etching bath in a concentration of from about 0.1% wt./vol. to about 7.5% wt./vol., and preferably from about 0.3% wt./vol. to about 3.0% wt./vol. (The designation wt./vol. is used herein and in the appended claims to indicate grams per ml. of final solution.)

The alkyl aryl polyether sulfonic acids which can be used in this invention can also be described as alkyl phenoxy polyethoxyethanol sulfonic acids. These acids have the general formula wherein R is an alkyl group having from about 7 to about 12 carbon atoms and n is an integer from about 2 to about 9. These acids can be made by condensing an alkyl phenol sulfonic acid with ethylene oxide. In a typical example of such acids the alkyl group is iso-octyl and n is 2.

In addition to the defined sulfonic acids per se, derivatives thereof which provide the ions of the acids in solution, such as the soluble metal salts, can also be used in this invention. Particularly preferred are the alkali metal salts, i.e., the lithium, sodium, and potassium salts. The salts can be used in the same manner and amounts as the acids themselves.

The aqueous sulfuric acid etching bath used in the process comprises from about 10% to about 40% wt./ vol. of sulfuric acid, and preferably from about 15% to about 30% wt./vol. At concentrations of acid higher than about 40% wt./vol., the etched surface smoothness is adversely affected and at concentrations lower than about 10%, both the surface smoothness and the rate of etching become inferior.

The temperature of the etching bath used in the process of the invention can vary from about room temperature (i.e., about 70 F.) to the boiling point of the solution (about 220 F). The portion of this range from about F. to about 180 F. is preferred.

it has been noted during tests of the process of this invention that the magnesium sulfate which inherently forms in the etching bath as a result of the reaction between magnesium and sulfuric acid acts as an etch-rate modifier and improves the surface appearance of the etched metal. Thus the etching action of a fresh bath initially containing no magnesium sulfate improves some- What as the sulfate concentration increases during the life of the bath up to a maximum concentration in the solution of about 70% wt./vol., and preferably about 50% wt./voi., expressed as MgSO -7H O. It is therefore advantageous to add magnesium sulfate to a fresh bath prior to use, or to incorporate in a fresh bath a portion of an exhausted bath containing a substantial quantity of the sulfate. It is not essential however, that magnesium sulfate be present initially in order to obtain the benefits accruing from the use of the sulfonated additives of this invention, since the action of these materials is entirely separate and distinct from that of the sulfate.

The advantages of the improved process of the invention are exemplified by a series of tests wherein magnesium panels were etched with sulfuric acid baths both in the presence and absence of the defined sulfonated additives. In these tests the etching bath comprised water, 25% wt./vol. sulfuric acid and 1% Wt./vol. of sulfonated additive. There was also present 20% wt./vol. of mag nesium sulfate. The temperature of the etching bath was about 180 F. After etching, the panel was measured for surface roughness and examined for appearance, tapering, and groove formation. The etching rate was also calculated. The results of the tests are given in Table I.

The foregoing data indicate the unexpected and unpredictable nature of the results obtained using the sui- Table I Surface Rourhncss, mirroinches Etch Rate, (0.03 cut-off) Additive Penetration, Remarks mills/min.

With Against Grain Grain None 6.0 Not acceptable. Tapcring. Groove formation. Surface bright. Xylene sulfouic arid 6.6 30 .35 Acceptable. N tapering. No pinholes. Surface smooth and brlt'ht. Toluene sulfonic acid 5.3 35 35 Acceptable. No tapering. Surface smooth and bright. Some pinholes. Sulfonatedoleic acid 0.3 40 45 Acceptable. No tapering. Surface smooth and bright. Some pinholes. Sodium salt ofulkylarylpolyether 4U 46 Acceptable. No taperin N0 sulfonic acid. i groove formation. Surface bright.

l Some pinholes. l

1 In which the alkyl group is iso-octyl, and the polyether group is formed of approximately two moles of ethylene oxide.

The data of the above Table I clearly demonstrates that the presence of the sulfonated additives of the invention improves the etching of magnesium with sulfuric acid solutions at elevated temperatures by eliminating tapering and groove formation and at the same time insuring the formation of a bright surface. Although the acid additive materials per so were used in the tests described above, equivalent results can be obtained by using the soluble metal salts, and preferably the alkali metal salts of these acids, at substantially the same level of concentration.

It has been determined that the defined sulfonated materials are unique in their ability to improve the etching action of sulfuric acid on magnesium and its alloys. Thus other similar sulfonic acids which were tested were not effective to inhibit defects such as tapering and groove formation. The following table illustrates the effect of some of these other materials. The data were obtained in a series of tests duplicating the conditions of the tests reported in Table I.

Table II Etch Rate, lrnctro tion, InillS/ min.

Additive Remarks Benzene sulfunic acid 3. 8 Not acceptable. Groove formation. Surface scaled and dull.

sealed and dull.

For convenience in manufacture, shipping, and storage, it is preferred to prepare the etching bath compositions of the invention in the form of concentrates containing more than about 40% wt./vol. of sulfuric acid and from about 0.25% by weight to about 20% by weight, based on the sulfuric acid, of the sulfonated material. Such concentrates, when diluted with water to a sulfuric acid concentration within the range from about 10% wt./vol. to about 40% wt./vol. will automatically have present an amount of sulfonatcd additive within the operative range. A preferred concentrate of this type consists of from about 80% to about 99% by weight of sulfuric acid with the remainder (i.e., from about to about 1% by weight, based on the total concentrate) being the sulfonnted material. This concentrate can be diluted to yield an etching bath composition in which the constituents are present in the preferred concentrations.

fonated materials of the invention. Although no explanation for the effectiveness of these materials can be given, the practical benefits attending the use thereof in the process of the invention have been demonstrated.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

We claim:

1. A method for etching a metal selected from the group consisting of magnesium and magnesium base alloys which comprises treating said metal with an aqueous etching solution comprising sulfuric acid and a small amount, sufficient to improve the etching characteristics of said solution, of a sulfonated organic material selected from the group consisting of xylene sulfonic acid, toluene sulfonic acid, alkyl aryl polyether sulfonic acids, sulfonated oleic acid, the soluble metal salts of said acids, and mixtures thereof.

2. The method of claim 1 wherein said etching solution contains from about 0.1% wt./vol. to about 7.5% wt./vol. of said sulfonated material.

3. The method of claim 1 wherein said treatment takes place at a temperature within the range from about room temperature to about the boiling point of said solution.

4. The method of claim 1 wherein said etching solution comprises from about 0.1% wt./vol. to about 7.5% wt./vol. of said sulfonated material and from about 10% wt./vol. to about 40% wt./vol. of sulfuric acid, and said treatment takes place at a temperature within the range from about room temperature to about the boiling point of said solution,

5. The method of claim 4 wherein said sulfonated material provides xylene sulfonate ions.

6. The method of claim 4 wherein said sulfonated material provides toluene sulfonate ions.

7. The method of claim 4 wherein said sulfonated material provides oleic acid sulfonate ions.

8. The method of claim 4 wherein said sulfonated material provides alkyl aryl polyether sulfonate ions.

9. A method for etching a metal selected from the group consisting of magnesium and magnesium base alloys which comprises treating said metal with an aqueous etching bath comprising from about 15% wt./vol. to about 30% wt./vol. of sulfuric acid and from about 0.3% Wt./vol. to about 3.0% wt./vo1. of xylene sulfonic acid at a temperature Within the range from about F. to about F.

10. An etching bath concentrate adapted to be diluted with water for use as an etching bath in etching magnesium and magnesium base alloys, said concentrate containing more than about 40% wt./vol. of sulfuric acid 5 and from about 0.25% by weight to about 20% by weight, based on said sulfuric acid, of a sulfonated organic material selected from the group consisting of xylene sulfonic acid, toluene sulfonic acid, alkyl aryl polyether sulfonic acids, sulfonated oleic acid, the soluble metal salts of said acids, and mixtures thereof.

11. The composition of claim 10 which comprises from about 90% to about 99% by weight of sulfuric acid and from about 0.5% to about 10% by weight of said sulfonated material.

12. The composition of claim 11 wherein said sulfonated material provides xylene sulfonate ions.

13. The composition of claim 11 wherein said sulfonated material provides toluene sulfonate ions.

14. The composition of claim 11 wherein said sulfonated material provides oleic acid sulfonate ions.

15. The composition of claim 11 wherein said sulfonated material provides alkyl aryl polyether sulfonate tons.

16. An etching bath comprising water, from about References Cited in the file of this patent UNITED STATES PATENTS 1,836,445 Chapman Dec. 15, 1931 1,918,545 Hoy July 18, 1933 2,287,050 Miller June 23, 1942 2,674,523 McDonald et a1 Apr. 6, 1954 2,698,781 Meyer Jan. 4, 1955 2,846,295 Patterson et a1 Aug. 5, 1958 OTHER REFERENCES Chemistry of the Metal Chelate Compounds by Martell and Calvin, copyright 1952, published by Prentice- 20 Hall, Inc., N.J., pp. 516, 541, 543, 544. 

1. A METHOD FOR ETCHING A METAL SELECTED FROM THE GROUP CONSISTING OF MAGNESIUM AND MAGNESIUM BASE ALLOYS WHICH COMPRISES TREATING SAID METAL WITH AN AQUEOUS ETCHING SOLUTION COMPRISING SULFURIC ACID AND A SMALL AMOUNT, SUFFICIENT TO IMPROVE THE ETCHING CHARACTERISTICS OF SAID SOLUTION, OF A SULFONATED ORGANIC MATERIAL SELECTED FROM THE GROUP CONSISTING OF XYLENE SULFONIC ACID, TOLUENE SULFONIC ACID, ALKYL ARYL POLYETHER SULFONIC ACIDS, SULFORNATED OLEIC ACID, THE SOLUBLE METAL SALTS OF SAID ACIDS, AND MIXTURES THEREOF. 